The technical field of the present disclosure generally relates to control of selective catalytic reduction (SCR) aftertreatment systems for internal combustion engines.
SCR systems present several control challenges for internal combustion engine applications, including for mobile applications. SCR systems include a reduction catalyst and a reductant, such as urea or ammonia. An injector provides the reductant to the exhaust stream at a position upstream of the reduction catalyst, and the reductant enters the gas phase of the exhaust stream as ammonia. A delay sometimes occurs between the introduction of the reductant and the availability of the reductant product, for example injected particles of the reductant may need to evaporate into the exhaust stream, hydrolyze from urea to ammonia, and/or thoroughly mix into the exhaust stream for general availability across the reduction catalyst. Additionally, the reductant catalyst may include some ammonia storage capacity. Storage capacity can complicate the controls process, for example, by creating additional controls targets (e.g. a storage target), by releasing ammonia unexpectedly (e.g. when a system condition causes a decrease in storage capacity), and/or by adsorbing some of the injected ammonia in an early part of the catalyst thereby reducing the availability of ammonia at a rear portion of the catalyst during catalyst filling operating periods.
The challenges presented by presently available SCR systems are exacerbated by the transient nature of mobile applications. The engine load and speed profile varies during operations in a manner that is determined by an operator and generally not known in advance to the SCR control system. Additionally, available feedback control systems suffer from several drawbacks. For example, the concentration of ammonia is difficult to determine in real time. Commercially reasonable NOx sensors can suffer from cross-sensitivity with ammonia, complicating the determination of the amount of NOx present in the exhaust gas outlet from the SCR catalyst. The addition of an NH3 sensor to the control system can improve control capabilities, but adds cost to the system.
Ammonia is generally an undesirable constituent of the final exhaust emissions, and ammonia that is emitted from, or “slips” from, the catalyst represents ineffectively utilized reductant that increases operating costs. Also, in systems that utilize an ammonia oxidation (AMOX) catalyst downstream of the SCR catalyst, ammonia slip can be converted to NOx in certain conditions. Therefore it is desirable to operate at a very low or zero ammonia concentration at the outlet of SCR catalyst. However, NOx sensors that are cross-sensitive to ammonia, and the conversion of NH3 slip to NOx in systems that employ an AMOX catalyst, hinder the ability to provide a reliable estimate of the occurrence and/or amount of ammonia slip. This reduces the effectiveness of feedback SCR control in providing an optimal amount of ammonia to the exhaust system and potentially creates false indications of an SCR and/or reductant injector fault conditions. As a result, further contributions in SCR control in response to ammonia slip conditions are needed.